[NXCM-5327] Do not aggregate content of P2 composite member repositories at root level

This will result in only showing the composite*.xmls and p2.index in root, and such, members artifacts/content.xmls to not be shown
[NXCM-5327] Implement a P2 client so we can easy manage repos in modern ITs
Making IT more reliable.

By inserting a new waitForAllTasksToStop. The
runTask should wait for the spawned bg task,
but it seems unreliable, and is some cases
the call does not block.

By having the 2nd call, we actually increase the
probability that server side task actually get
running, hence, be detected (and block IT main thread)
and actually be awaited to finish it's job.
Merge remote-tracking branch 'origin/master' into nexus-5418-md-helper-fix
NEXUS-5418: More general fix

Instead fixing all the code that would delete
maven artifact checksums, handle this
change at more general level: at
maven repository level.

With this change, existing code works
"as before", as maven repositories will
stop throwing INFEx at checksum deletion
attempt, while the coll#list still
contains those.